Weston



March 22, 1955 D. WESTON 2,704,636

ROTATABLE DRUM MILL WITH ANNULAR END WALL MEMBERS EACH HAVING FRUSTO-CONICAL. DEFLECTING SURFACES THEREON Filed Oct. 18, 1952- 2 Sheets-Sheet l m m f 4 5 0 32 J w4 6.6. 3 w 5 7 m u:

Mush T02 DAL/I0 WESTON D. WESTON ROTATABLE DRUM MILL WITH ANNULAR END WALL MEMBERS EACH HAVING FRUSTO-CONICAL DEFLECTING SURFACES THEREON Filed Oct. 18, 1952 2 Sheets-Sheet 2 5 3w 2 3 10 um mm VE 0F Nl/E N TOR 3 /111 1) M5 TON 6 United States Patent ROTATABLE DRUM MILL WITH ANNULAR END WALL MEMBERS EACH HAVING FRUSTO- CONICAL DEFLECTING SURFACES THEREON David Weston, Toronto, Ontario, Canada Application October 18, 1952, Serial No. 315,470

6 Claims. (Cl. 241-183) This invention relates to material reduction mills of the type comprising a rotatable drum arranged to rotate about a horizontal axis and receive material to be reduced through an axial feed port concentric with the axis of rotation of the drum. This appllcatlon IS a continuation-in-part of my co-pending appllcatlon Serial No. 175,353, filed July 22, 1950, now abandoned.

In my prior Patent No. 2,555,171 issued May 29, 1951, I describe a mill of the above type wherem the drum, having a diameter of at least twice its length was provided with highly upstanding crusher bars arranged transversely of the drum and secured at spaced intervals around the inner periphery thereof. The drum was characterized by the provision of frusto-conical deflectmg surfaces formed on annular liners mounted upon and projecting inwardly from each end wall of the drum and so disposed as to deflect feed material and oversize return material to cause it to be delivered substantially centrally to the cylindrical wall at the bottom of the drum. While the arrangement described in my said patent operates satisfactorily, l have found that material increases in capacity and operating efficiency may be obtained by the provision of further operative annular elements disposed radially outwardly from sa d first mentioned surfaces and arranged to work w1th1n the charge of material undergoing comminution 1n the drum so as to produce a novel effect, hereinafter referred to as a keying action within the charge, whereby the crushing action of the mill is materially increased, and the grinding efiiciency is enhanced by the provlsion of greater compactness in the charge.

As will be appreciated, the crushing action in a mill of the type described in my said patent takes place malnly in the bottom regions of the mill by virtue of the crusher bars driving into the toe of the charge, and crushing the material in the toe, or fresh feed material which has dropped down in front of the toe, against a backing formed by the rest of the charge. Since the charge consists of a large number of particles of partially reduced material, each of which is moving in relation to every other, and since the top of the charge is unconfined, the charge does not provide a very solid backing against which the crusher bars may crush the material in the toe in the manner outlined above. There is therefore a limit to the amount of useful energy which may be transmitted to the charge by the crushing action outlined.

According to the present invention the effectiveness of this crushing action is substantially increased by the provision on each of the end walls of the drum of an annular member having formed thereon a frusto-com'cal surface facing the cylindrical wall of the drum and inclining away therefrom. These members are positioned so that the frusto-conical surfaces upon them pass completely into the charge and work within it during operation of the mill. In order to work effectively within the charge, these members will each extend inwardly into the drum an axial distance which is at least ten percent of the length of the drum.

The frusto-conical surfaces which are the principal feature of the present invention may be formed on the radially outer sides of the same annular members which are provided with the deflecting surfaces described in my said co-pending patent, in which case there will be a tremities of the two frusto-conical surfaces, or alternatively they may be formed on separate annular members disposed radially outwardly of the annular members carrying said deflecting surfaces.

The invention and its operation will be described in greater detail with reference to the accompanying drawings wherein:

Figure 1 is a vertical longitudinal section through a mill embodying one form of the invention,

Figure 2 is a diagrammatic longitudinal section of a mill illustrating an alternative embodiment of the invention,

Figure 3 is a diagrammatic longitudinal section of a mill illustrating the invention as embodied by the mill illustrated in Figure 1,

Figure 4 is a diagrammatic view of the interior of the drum of a mill illustrating the general distribution and paths of travel of particles of material undergoing reduction when the drum is being rotated at a speed of from about 82 to 92% of critical speed with the normal charge volume for this type of mill i. e. about 20 to 32% of the drum volume, and

Figure 5 is an enlarged detail view of the false toe and true toe zones in the charge of a mill during operation.

Referring in detail to Figure 1 it will be observed that the mill illustrated is in general similar to the mill described in my said patent and comprises a drum 10 which is arranged for rotation about a horizontal axis. As will be observed, the drum 10 is short in relation to its diameter. The minimum practical diameter: length ratio for a mill of this type is about 2:1, and normally the ratio will vary depending upon the size and design from about 2.5:1 to about 3.5:1 or higher.

The shell of the drum 10 is formed from the cylindrical wall 11 and the end walls 12 and 13 which are provided with central openings to accommodate the hollow trunnions 14 and 14a to which the end walls 12 and 13 are secured. The hollow trunnions 14 and 14a are mounted for rotation in suitable aligned bearings 15 and 16 respectively, which rest on suitable foundations 17 and 18. The mill is driven by the drive gear 19 which surrounds the periphery of the end wall 12. The drive pinion and power source, being entirely conventional, are not shown.

Feed material, which may be run-of the mine up to a nominal size of 18" in its largest dimension, depending upon the size of the drum, is fed to the mill on conveyor 20 and enters the drum through the feed chute 21 and inlet opening 22.

Reduced material is withdrawn through the outlet opening 23 in a current of air which is drawn down through feed chute 22, across the drum and out through outlet opening 23 and conduit 24 by an exhaust fan (not shown).

Inside the drum 10, the highly-upstanding transverse crusher bars 25 are secured at spaced apart intervals about the inner periphery of the cylindrical wall 11. The crusher bars 25 are spaced sufficiently widely apart that pieces of material undergoing comminution in the mill cannot become wedged between them, and their inward projection is considerable so as to enable them to crush the larger piece of material which are fed into the mill against the charge when they are driven into it as the drum is rotated. The cylindrical wall 11 is protected from wear by suitable liner elements 25a secured between adjacent crusher bars.

Surrounding the inlet and outlet openings 22 and 23 respectively and secured respectively to the end walls 12 and 13 of the drum are the annular members 27 and 28. As will be seen, these members are generally triangular in cross-section and carry the frusto-conical deflecting surfaces 29 and 30 facing the axis of rotation of the drum and which are as defined in my said Patent No. 2,555,171.

Surrounding the abovementioned annular members 27 and 28, respectively and also secured respectively to the end walls 12 and 13 of of the drum are the annular members 31 and 32, which are of generally triangular cross-section similar to that of the elements 27 and 28. The principal feature of the present invention is the range of 82-92% of the critical speed, that is to say i at a speed which is 82-92% of that speed which would be suflicient to carry all particles in the mill of an effective diameter of 3" or less completely around the mill against the periphery thereof under the influence of centrifu al force. The critical speed of any drum type mill is defined in Taggart (Handbook of Mineral Dressing 1945, sect. 5.05) as being where r is the radius of the mill as measured along a idiameter joining two diametrically opposed crusher bar aces.

With the speed of rotation of the drum adjusted as aforesaid, material is fed to the mill at a suitable rate to maintain a normal charge (i. e. occupying about 2032% of the drum volume) within the mill, and reduced material is withdrawn as it comes within a selected size range by entrainment in the air stream passing through the niil, and is carried away from the mill through conurt With the mill operating in this manner, the general distribution of particle sizes within the mill and their paths of travel will be substantially as illustrated in Figure 4. The body of the charge will be sitting up against the rising side of the drum bounded approximately by the heavy dotted line a, with toe b situated in the region of a plane passing vertically throu h the axis of rotation of the drum. Material fed to the mill will fall into the bottom of the drum iust in front of the toe of the charge, and will form a false toe c as illustrated. As the crusher bars advance into the charge, they will crush the pieces of material in the false toe against the toe b backed by the rest of the body of the char e. forcing them, as they are being reduced by the crushing action, throu h the true toe b and into the main body of the charge. Here, due to the wellknown induced couple which makes all bodies within a mill tend to rotate on an axis parallel to the axis of the mill, the particles of material will rotate on their own axes and abrade one on the other, whereby they are further reduced by grinding.

All of the particles within the charge will sooner or later tend to move away from the periphery of the drum as the influence of gravity overcomes the centrifu al force tending to hold them against the periphery. The first to move away will be the larger particles, since their centres of gravity will of necessity be farther from the periphery, and consequently their moment arm about the axis of the mill is shorter than that of the smaller particles. The coarser particles thus migrate towards the centre of the mill almost immediately after being driven into the char e in the area designated in Figure 4 as the coarse particles migration zone. As they leave the periphery, their rotation about the mill axis is reduced, and some drag will take place in the drag zone between the migrating particles and th se remaining adiacent the periphery. At the top of the charge in what is desi nated the "abrasion zone medium sized particles are moving away from the periphery, and due to the induced couple previously referred to all are rotating on their own axes, and thus grinding each other and particles of finer material caught between them. Particles of size greater than about 1" will, on reachin the inner extremity of the body of the ch r e cascade by ravit throu h the cascade zone and return to the toe b of the char e. while particles of Under 1" diamet r wi l leave the top of the ch rge in a generallv parabolic p th and f ll through the cataracting zone t strike the bare f ces of the downwardl movin crusher bars on the other side of the drum. and be carried back into the charge. Fines in the feed. and oversize returning throu h the outlet of the mill will flow over the deflecting surfaces 29 and 30, being carried slightly around by their rotation, and deposit on the top of the cascade zone from where they are carried into theatloe b of the charge by the descending cascading materi The above general explanation of the various actions taking place within the drum during operation of the mill is true of all mills of this type whether or not equipped with annular members arranged to produce the keying effect which is the principal feature of the present inventron.

The effect of the presence of the annular members 31 and 32 and their frusto-conical surfaces 33 and 34 is two-fold. Firstly, as the material in the charge moves away from the periphery, the frusto-conical surfaces 33 and 34 react upon them in a substantially normal direction as indicated by the arrows 40 and 41 in Figure 1. This reaction is carried from particle to particle in a manner tending to form bridges between the two surfaces 33 and 34 resisting the movement of material towards the axis of the drum. The result is that the charge is held together as a more compact mass and the particles are enabled to do more grinding work one on the other as they move relative to one another in the manner previously explained.

The most important effect, however is in relation to the crushing action of the mill. As each crusher bar drives into the toe of the charge, the momentarily increased force tending to displace the material already in the charge to make room for the crusher bar and material driven by it, momentarily increases the forces tending to move particles of material towards the drum axis. This increases the reaction of the surfaces 33 and 34 and increases the strength of the bridging action. As is well known in phenomena associated with the bridging of particulate material, the greater the force tending to break through the bridge, the more solid the bridge becomes. Thus when the crusher bar moves into the charge, the charge becomes momentarily keyed as a solidified, compact mass against which the material driven by the crusher bar is crushed, as against the stationary jaw of a jaw crusher. Moreover, the great force momentarily tending to break the bridges formed in the material as aforesaid as each crusher bar drives into the toe of the charge produces a secondary crushing action along the span of the bridges, causing these bridges to continually break and reform. In the result therefore considerably more useful energy can be put into the mill, and the capacity of a mill of any given size is increased.

It should be explained that the bridging referred to above actually takes place over considerable radial depth, for at its deepest point 42, the charge will actually cover the radially outward faces 43 and 44 of the annular elements 27 and 28 and bridging will occur between them. The zone where bridging is occurring will therefore extend radially from just inwardly of the crusher bars to just outwardly of the apices 45 and 46 of the annular members 27 and 28.

In cases where the mill is designed to treat a relatively fine feed material, the annular members 27 and 28 may be formed integrally with the annular members 31 and 32 in the manner illustrated in Figure 2, with the vertical annular flats 47 and 48 extending between the deflecting surfaces 29 and 30 and the frusto-conical surfaces 33 and 34. Once initiated by the surfaces 33 and 34. the bridging zone will extend upwardly between these flats, the shorter distance between the annular flats 47 and 48 permitting the bridging of smaller particle size material than would bridge satisfactorily in the arrangement shown in Figure 3.

In order to bring about satisfactory bridging conditions and provide the keying action explained above. certain dimensional limitations are imposed on the design and arrangement of the annular members taking part in the action. Firstly, the apices of the annular members carrying the frusto-conical surfaces 33 and 34 must each extend inwardly into the mill interior a distance not less than 10% of the mill length. They may extend inwardly up to about 25% of the mill len th but it is not recommended that they be desi d to extend inwardly appreciably more than about 15% of the mill length because the volume of mill occupied by the annular members 31 and 32 will in that case be in materially to reduce the volume available in the mill for charge. Practical experiments ha e indicated that to obtain a maximum of keying effect without appreciably reducing the space in the mill available for change, annular keying elements which extend inwardly about 15% of mill length are to be preferred.

Secondly, while the angle of inclination of the frustowill vary within this range depending upon the actual dimensions of the mill drum. Table I sets forth preferred ratios for 1 for various sizes of mills.

Table 1 Dimension Ratio m2 Diameter,

feet m1, inches Length, Diameterzlength feet ratio An indication of the increase in capacity effected by reason of the keying action brought about according to the invention is given in the following examples wherein comparative tests were carried out in a mill having nominal dimensions of 5 feet in diameter by 2 feet in length. A sample of material 1 was divided into two aliquot portions. One portion was run through the mill, which was equipped with the annular members of the invention in accordance with the embodiment illustrated in Figure 1. The outer annular member was then removed so that the mill was in accordance with the teachings of my prior Patent No. 2,555,171 and the other aliquot portion of material was run through the mill. The results were tabulated as follows:

Capacity,

Equipmen lbs/hr.

What I claim as my invention is:

1. A material reduction mill comprising a rotatable drum defined by two end walls and a cylindrical wall, said drum having a diameter length ratio of at least 2:1 and being provided with a plurality of substantially parallel transverse crusher bars projecting from the cylindrical wall inside the drum and spaced so widely apart that practically no wedging of material between the crusher bars may take place; an annular member arranged to work within the charge during normal operation of the mill projecting from each end wall inside the drum, a distance equal to at least of the length of the drum and whose surface facing the cylindrical wall is substantially frusto-conical and forms an angle with the adjacent end wall which is substantially greater than 90, and the ratio of the distance between the apices of the annular members to the distance from the apices of the annular Magnetic tailings from the manufacture of fused aluminium ox ide consisting of a fused mixture of term-silicon and aluminium oxide-s ze range-6"--very hard dense material.

members to the inner faces of the crusher bars being from about .40 to about 1.5.

2. The combination defined in claim 1, in which the said surface on each said annular member forms an angle of approximately 120 with the vertical walls of the drum on an axial longitudinal section thereof.

3. The combination defined in claim 1 in which each said annular member extends inwardly towards the other a distance which is about 15% of the length of the drum.

4. The combination defined in claim 1 in which said annular members are formed with a second substantially gusto-conical surface facing the axis of rotation of the rum.

5. The combination defined in claim 1 in which the drum has centrally located inlet and outlet openings and each said annular member is formed with a substantially flat annular surface intersecting said surface facing the cylindrical wall of the drum at the inner margin thereof and being substantially parallel to the vertical wall of the drum, said annular member also having a substantially frusto-conical surface facing and inclining away from the axis of rotation of the drum and extending between said substantially flat surface and the vertical walls of the drum in the region of said centrally located inlet and outlet openings to the said drum, said substantially frusto-conical surface being disposed in the trajectory of feed entering said mill to prevent segregation thereof according to particle size.

6. A material reduction mill comprising a rotatable drum defined by two end walls and a cylindrical wall, said drum having a diameter length ratio of at least 2:1 and being provided with aligned axial inlet and outlet ports respectively, for the entry thereinto of feed material and air and the discharge therefrom of air and entrained particles within a selected size range, said drum also being provided with a plurality of substantially parallel crusher bars projecting from the cylindrical wall inside the drum and spaced so widely apart that practically no wedging of the material between the crushing bars may take place; an annular member arranged to work within the charge during normal operation of the mill projecting from each end wall inside the drum, a distance equal to at least 10% of the length of the drum and whose surface facing the cylindrical wall is substantially frusto-conical and forms an angle with the adjacent end wall which is substantially greater than and the ratio of the distance between the apices of the annular members to the distance from the apices of the annular members to the inner faces of the crusher bars being from about .40 to about 1.5; a second annular member concentric with and radially inward of said first-mentioned annular member projecting from each end wall inside the drum a distance equal to the distance to which said first-mentioned annular member projects and whose surface facing the axis of rotation of the mill is substantially frusto-conical and is disposed in position to deflect feed material entering through said feed port on the one hand and oversize returing to the drum through said outlet port on the other hand so as to deliver the same substantially centrally to the cylindrical wall at the bottom of the drum.

References Cited in the file of this patent UNITED STATES PATENTS 413,388 Coward Oct. 22, 1889 1,307,952 Ball June 24, 1919 1,335,269 Ball Mar. 30, 1920 1,690,493 Marcy NOV. 6, 1928 1,872,036 Hardinge Aug. 16, 1932 2,274,331 Howes Feb. 24, 1942 2,555,171 Weston May 29, 1951 FOREIGN PATENTS 7,801 Great Britain Mar. 29, 1911 452,305 Germany Nov. 8, 1927 

